The invention relates to an aluminosilicate glass for optical applications which in particular can be used advantageously as a core glass in optical fibers.
Prior art optical glasses having a refractive index between 1.55 and 1.65 (light barium flint region and extra dense crown region) usually contain PbO to reach the desired optical characteristics (refractive index of 1.55≦nd≦1.65 and an Abbe number of 48≦vd≦57) and a good transmission. Such glasses are of interest for numerous optical applications, e.g. for applications in imaging, projection, telecommunication, optical communication engineering and laser technology, however, in particular also for fiber applications (imaging fibers and/or light transmission fibers). Due to their lead content such glasses offer a low chemical resistance. Also often As2O3 is used as a refining agent. Since within the last years the glass component PbO and often also As2O3 has been regarded as environmentally problematic, the most manufacturers of optical instruments and products tend to use glasses free of lead and arsenic. For the application in products having a high degree of coating also materials of enhanced chemical stability (for undergoing the coating processes without damage) by keeping a high transmission (very low loss) are gaining more and more importance.
For replacing lead in classical optical glasses, glasses containing large amounts of TiO2 in a silicate matrix are known that lead to glasses being very instable with respect to crystallization, being difficultly meltable and hardly processable. In addition, the transmission of the glasses deteriorates (the loss increases) due to the intrinsic adsorption of TiO2.
Also lately the development of “short” glasses has been desired due to processing aspects, i.e. glasses the viscosity of which is extremely temperature dependent. This behavior during processing offers the advantage that hot forming times, i.e. the times of mold closure, can be reduced. Thereby throughput can be enhanced on the one hand, and on the other hand it is easy on the mold material, this having a very positive effect on the total manufacturing cost. Also due to the fast cooling (shorter mold closure times) even glasses exhibiting an enhanced crystallization tendency can be processed when compared with longer glasses, and an initial nucleation which would be problematic during subsequent secondary heat-forming steps (fiber drawing, ion-exchange, subsequent pressing, reforming, fine cooling, etc.) is avoided.
For micro structuring purposes (gradient-index lenses, (flat, rod shaped fiber (bundle)-like) light guides, spherical lenses etc.) using ion-exchange (e.g. (Na—Ag)) the novel materials should also be ion-exchangeable on an economical basis in standard processes. An additional characteristic may be the tension building ion-exchangeability (e.g. Na—K, “pretensioning”).
Optical materials for general applications should also be applicable in fiber applications, e.g. as fiber core glasses. To this end, novel types of glasses must particularly be tolerant against secondary heat-forming steps (fiber drawing, melting together, etc.) and must have a good compatibility with conventional fiber cladding glasses.
Commonly, a glass fiber for the transmission of light comprises a highly refractive core glass and a cladding glass enclosing the core glass and having a lower refractive index than the core glass. Under suitable conditions, stepped-index fibers comprising a core glass and a cladding glass completely enclosing the core glass at its outer peripheral wall can be produced. A light transmissive glass body of fiber shape within which the core glass offers a constant refractive index across the total cross surface is referred to as a stepped-index fiber. Glass fibers of this type transmit light, being introduced at one end of the fiber into the core, to the other end of the fiber, wherein the light is completely reflected at the interface between the core glass and the cladding glass (total reflection).
The amount of light that can be coupled into and transmitted within such a fiber is proportional to the square of the numerical aperture (NA) of the fiber and to the cross-sectional area of the fiber core. For transmitting large light amounts via long or middle distances (≦100 meters), such stepped-index fibers are often packed together to dense fiber bundles, are equipped with a protecting hose, are bonded with their ends to metal shells, and the front surfaces are processed to yield optically flat surfaces by grinding and polishing. Suitably fabricated optical fiber bundles are referred to as fiber optical light guides. In case a production process is used which allows for a geometric arrangement of individual fibers, in this way image light guides can be produced.
The higher NA of the individual fibers within the bundle, the larger amounts of lights can be transmitted by these light guides.
Fiber optical light guides are used in various technical and medical applications (general industrial processes, illumination, traffic, automobile, architecture, endoscopy, dental medicine). Their most important function is the transmission of a light stream as large as possible from a place A to another place B, normally via short or middle distances (a few to 100 meters maximum). Herein often light emerging from a high power light source, such as a halogen or discharge lamp, is coupled into the fiber bundle by means of optical aids, such as a lens or a reflector.
The light amount transmitted by fiber optical light guides depends, apart from NA of its fibers, also from the transmissive characteristics of the core glasses contained therein. Only core glasses of very specific compositions having very low contaminations in the raw materials, from which they are molten, transmit the light with low attenuation along the total length of the light transmitter. The raw materials for melting such core glasses are relatively expensive due to the high purity required which may lead to considerable manufacturing costs for such fibers or for such light guides made thereof.
Apart from the amount of light transmitted by a fiber optical light guide, also a color true transmission of the light is of importance in many cases. Due to the spectral transmissive dependence of the core glass which is contained in the fibers, there may be a color deviation in the color position of the feeding light source, which may have a higher or lower degree, this often leading to a yellow color cast of the light emerging from the light guide. This is always detrimental when a color neutral representation is required (e.g. in the medical endoscopy with photographical image documentation for differentiating between healthy and malignant tissue etc.). The manufacture of optical stepped-index fibers from multi-component-glasses is performed either in the so-called double-mold process or in the rod-tube process. In both cases, the core and cladding glasses are heated up to temperatures which correspond to a viscosity range between 104 and 103 dpas, and are drawn to fibers. To allow a manufacture of a stable fiber with low loss, the core and cladding glasses must be compatible to each other with respect to a variety of characteristics, such as the course of viscosity, the thermal expansion, the crystallization tendency, etc. In particular, there may be no contact reaction or crystallization, respectively, at the interface between the fiber core and cladding which would considerably impair a total reflection of the light introduced into the fiber core and which would render the fiber unsuitable for an application for low-loss light transmission. In addition, also the mechanical stability of the fiber would be negatively influenced by crystallization.
From DE 27 47 919 A1 a photochromic glass is known which predominantly comprises silver oxide or alternatively at least copper oxide. Such photochromic glasses change their spectral transmission characteristics toward lower transmissivities under the influence of electromagnetic radiation in the visible spectral range. The addition of silver and possibly of copper leads to a considerably deteriorated transmission characteristic and to an unnecessary cost increase of the glasses for the desired applications.
Another phototropic glass is known from DE 1 924 493 C3. The respective glass comprises a high amount of B2O3 which is between 30 and 80 mol-%. This high boron content decreases the crystallization stability of the glasses due to an extreme decrease in viscosity, since the migration velocity of crystal forming components is increased. Also the boron content increases the aggressiveness against refractory material so that production costs are largely increased by shorter melting pot lives. Also La2O3 having a high intrinsic absorption and thereby being detrimental with respect to attenuation, must be added by at least one mol-%.
Optical glasses having a highly anomalous partial dispersion are known from JP 98 130 033 A. Also these glasses have a high content of at least 12.5 wt.-% of B2O3. In addition they contain by obligation at least 6 wt.-% of the very expensive raw material Nb2O5 for reaching the claimed optical characteristics.
Also the glasses disclosed by JP 91 037 130 A (patent abstracts of Japan 03 037 130 A) must be considered in a comparable way. Also here, apart from the mandatory boron trioxide content of at least 11 wt.-% there is an addition of at least 4 wt.-% of La2O3 and/or of Gd2O3 for reaching particular optical characteristics, this leading to an increase in loss. Also a mandatory amount of at least 3 wt.-% Li2O is detrimental, this leading to an additional increase in aggressiveness of the glass against the refractory material, even increasing together with the existing B2O3 in a synergistic way.
Also, the optical glasses known from JP 85 221 338 A (patent abstracts of Japan 60 221 338 A) must be evaluated in a similar way. The mandatory B2O3 content of at least 1 wt.-% and of 0.5 wt.-% of Li2O enhances crystallization and decreases the refractory material resistance, while the mandatory amounts of at least 1 wt.-% La2O3 and at least 0.1 wt.-% of Y2O3 necessary for achieving the optical position decrease the attenuation characteristics of the material by intrinsic absorption while simultaneously increasing the cost of the glass.
From JP 89 133 956 A (patent abstracts of Japan 01 133 956 A) also a glass for the manufacture of index-gradient lenses is known having a B2O3 content of up to 20 wt.-%, this also having the same drawbacks.
The glass known from JP 87 012 635 A (patent abstracts of Japan 62 012 635 A) must be considered in a similar way. This glass comprises a mandatory amount of 10 mol-% of Li2O and an optional amount of boron trioxide of up to 8 mol-%. Also at least 3 mol-% of Nb2O5 are added for reaching the optical characteristics.
JP 87 055 761 B also discloses glasses intended for ion-exchange and thereby for the manufacture of index-gradient lenses. Herein tantalum is used for ion-exchange which, from a current point of view, leads to an increased danger potential in excavation, extraction, purification and processing several orders higher than incurred with lead.
The same argumentation holds true with respect to the glasses known from DE 3 217 897 A1 which comprise at least 1 wt.-% of thallium.
The glasses known from DE 3 016 116 C2 have mandatory amounts of BaO which are between 30 and 45 wt.-% and also comprise at lease 5 wt.-% of boron trioxide. Herein on the one hand the position of the refractive index is shifted to refractive index values and dispersions higher than desired due to the high barium oxide contents. Also alkaline earth oxides at high amounts tend to produce diffusion barrier layers within the glass system and thus counteract an effective, economical ion-exchange process. Also they lower the crystallization stability of the glasses, in particular with combination with ZrO2 (mandatory compounded content BaO+ZrO2>38 wt.-%). Also the aggressiveness against refractory material is increased by the addition of boron trioxide.
Also glasses suitable for index-gradient lenses are disclosed by JP 87 012 633. Herein, an ion-exchange process with cesium-zinc is described. Cs2O3 with a mandatory content of at least 2.86 wt.-% is an extremely expensive raw material which does not have any function which could not also be overtaken by other alkali metal oxides, apart from adjusting the necessary ion-exchange characteristics according to this publication.
From GB 2 233 781 A further optical glasses for an achromatic lens system are known which consist of SiO2, R2O and ZrO2, wherein R2O is selected from Li2O, Na2O and K2O and the components comprise at least 30 wt.-% of the glass, wherein the rest shall be selected from compatible components, having a refractive index of 1.70 maximum and an Abbe number of less than 55. However, practically this application only discloses glasses comprising at least 2.6 wt.-% Li2O or at least 7 wt.-% PbO or ZrO2 additions of at least 10 wt.-%.
While the addition of PbO is detrimental for the chemical stability, the addition of Li2O impairs the refractory material stability. However, too high amounts of ZrO2 may lead to increased crystallization tendency.
From JP 77 045 612 A an optical flint glass is known, i.e. a glass having a higher refraction index with increased dispersion. Herein, it is mandatory to add 5 to 60 wt.-% Nb2O5 to achieve the desired optical characteristics. This is an expensive raw material, in particular in optical qualities, which should be avoided, if possible.
For the manufacture of stepped-index fibers in the prior art basically three fiber systems are known.
The fiber system which is probably the best-known and most common one comprises a core glass of high lead content (commonly 35 wt.-% PbO and an alkali borosilicate glass as a cladding glass. The advantage rests in the high numerical aperture that can be reached (up to more than 0.7 with PbO contents of >50% in the core glass) together with low manufacturing costs and a good capability of drawing to fibers without crystallization problems. This, however, is in contrast to drawbacks such as a mediocre or bad attenuation (200 to 300 dB/km) as well as a somewhat high color cast, mainly caused by Pb self absorption (blue edge of the visible spectral range), as well as dragged-in contaminations of elements highly blue coloring, such as chromium and nickel. Also lead as an environmentally polluting material has come into disrepute more and more and hence is applied for fibers only in specific applications or not at all any more.
A second fiber system comprises an alkali borosilicate glass which is applied as a core as well as a cladding glass.
In the prior art several such glass systems are described, e.g. within EP 0 018 110 B1 or in EP 0 081 928 B1 or in DE 29 40 451 A1 or in U.S. Pat. No. 4,264,131. Partially these glasses, apart from a high boron content, also contain high amounts of alkaline earth and/or zirconium and germanium oxide to reach the desired high refractive index. The advantage rests in the very low attenuation (partially around 10 dB/km) and in their low color cast together with normally environmentally friendly raw materials. A disadvantage of these glasses rests in the commonly lower numerical aperture of the fibers as well as in a lower chemical stability. Also the mandatory boron oxide amount (U.S. Pat. No. 4,264,131, EP 0 081 928 B1, DE 29 40 451 A) is detrimental with respect to the refractory material stability. Due to the lower chemical stability the fibers, during their manufacture, directly after drawing, e.g. from a drawing die at the double mold, must be supplied online with a plastic coating protection against possible chemical and/or mechanical attack. In addition, the low attenuation is achieved only by utilizing highly pure and thereby very expensive raw materials. The two last mentioned aspects, high manufacturing cost and a mandatory plastic coating, thus render practically impossible an application as fiber bundles for broader applications. By contrast, they are used as single fibers for data or energy transfer (laser fiber) in a variety of special applications.
Also fibers on pure Si2O-basis basically are possible as a third fiber system for fiber bundles for the transmission of light. Their advantages resting in an extremely low attenuation (up to 6 dB/km) in a good color neutrality and good environmental compatibility, are in contrast in particular to the high cost. Pure silica glass is extremely expensive due to its high processing temperature. In addition, there is a complicated doping process of the so-called preform according to which by the introduction of fluorine into the surface of a cylindrical rod the necessary reduction in refractory index of the pure quartz is reached that is necessary as an optical isolation to achieve light transfer in the later fiber. Also the numerical aperture of quartz fibers that can be reached is somewhat limited (0.22).